This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. Section 119 from an application for HANDOFF SUPPORTING APPARATUS AND METHOD IN CDMA COMMUNICATION SYSTEM SUPPORTING MULTI-FREQUENCY ASSIGNMENT filed earlier in the Korean Industrial Property Office on Mar. 16, 1999 and there duly assigned Serial No. 8711/1999.
1. Field of the Invention
The present invention relates to a handoff supporting apparatus and method in a code division multiple access (CDMA) communication system which supports the multi-frequency assignment, and more particularly to an apparatus and method for supporting the hand-off involving the multi-frequency assignment to a pico base transceiver station (BTS).
2. Description of the Related Art
An outdoor pico-base transceiver station (BTS) is normally installed in a special area such as downtown, a microcell, a local road, or a district town as a small-capacity BTS for serving the area where a small number of subscribers exist. The pico-BTS is implemented since it is difficult to have an effective arrangement of the BTS for the whole service area using only the large-capacity BTSs. However, the pico BTS currently implemented in the service cannot provide a multi-frequency assignment (multi-FA) service. Thus, a call which is not tuned to a primary frequency assignment, but to other frequency assignment coming from the neighboring BTSs, is always disconnected upon entering the pico-BTS area.
FIG. 1 is the circuit diagram of a transmitter in the conventional pico-BTS. Referring to FIG. 1, the transmitter includes a phase locked loop (PLL) synthesizer 1 for synthesizing the frequency FA1 under the control of the input terminal signal of an PLL, and an up-converter 2 for modulating the transmission data at the frequency FA1 synthesized by the PLL synthesizer 1.
The PLL synthesizer 1 synthesizes the corresponding frequency using the input signals of CLOCK, DATA, STROBE, AND REFERENCE of the PLL provided therein, and the up-converter 2 receives the frequencies synthesized by the PLL synthesizer 1 to modulate the transmission data accordingly. Meanwhile, the frequency signal filtered by a band pass filter (BPF) provided in the up-converter 2 is inputted and amplified by a power amplifier to be transmitted through an antenna.
FIG. 2 depicts a block diagram illustrating the construction of the conventional frequency assignment (1FA) system. In FIG. 2, the conventional 1FA system comprises a pico-cell main unit (PMU) 10 and a pico-cell radio unit (PRU) 20. According to the conventional 1FA system, a maximum spacing distance of 100 m is possible between the PMU 10 and the PRU 20. Moreover, one PRU 20 is required when the cell is provided with 1FA/Omni, two PRUs 20 are required in the case of 1FA/2sec, and three PRUs 20 are required in the case of 1FA/3sec. Currently, in the case of constructing the 1FA/OMni system, a modification up to 3FA/Omni is possible but not over 4FA.
FIG. 3 depicts a view illustrating the result of handoffs according to the conventional system. FIG. 3 illustrates that all mobile terminals, except for the mobile terminal having the primary frequency assignment, experience call disconnection due to the lack of frequency assignment when they enter from the ground level to the underground level. For example, if a mobile station tuned to the frequencies 2FA and 3FA but not to the primary frequency (1FA) moves from the ground level to the subway level, call disconnection is unavoidable as the pico-BTS installed in the subway only supports the primary frequency FA1.
As described above, as the conventional pico-BTS only supports the service in the primary frequency assignment, any mobile terminal with a different frequency assignment experiences a call disconnection when the terminal moves to the underground area or specific topography.
Accordingly, the present invention has been made in an effort to solve the problems occurring in the related art, and an object of the present invention is to provide a hand-off supporting apparatus and method in the code division multiple access (CDMA) communication system for supporting a multi-frequency hand-off to a small-capacity base transceiver station which only supports the primary frequency assignment.
Another object of the present invention is to provide an apparatus and method in the CDMA communication system which enables the small-capacity base transceiver station to transmit the frequency signals other than the primary frequency in a time hopping mode.
In order to achieve the above objects, according to the present invention, there is provided an apparatus for a small-capacity BTS having only the primary frequency assignment in the CDMA system which provides the pilot signals corresponding to a multi-frequency assignment (FA). The apparatus includes a first phase locked loop (PLL) synthesizer having a first PLL for synthesizing the primary frequency assignment (FA1) signal by controlling the first PLL; a second PLL synthesizer having a second PLL for synthesizing the multi-frequency assignment (FA) signals by controlling the second PLL; and, an up-converter for modulating the transmission data with the primary frequency assignment signal provided from the first PLL synthesizer, and for producing pilot signals according to the multi-frequency assignment and modulating the received signal with the multi-frequency assignment signals provided from the second PLL synthesizer prior to the RF modulation.